What's the target for solving climate change?

A man watches a screen showing a counter constantly updating the greenhouse gases in the Earth's atmosphere during sideline activities of the United Nations Framework Convention on Climate Change (COP-16), at the Business Action for Climate venue in Cancun, Mexico on December 4, 2010. Photograph: Cris Bouroncle/AFP/Getty Images

Climate change targets can be highly confusing. There are various types of targets, and lots of uncertainty about how each target could be met and what would happen if it was. Let's look at the three main types of target in turn.

Temperature targets

Ultimately, temperature rise is the thing that matters, as warming causes all the other symptoms of climate change. There's no agreement about what constitutes a "safe" temperature rise but in the hope of reducing the risks to an acceptable level – and in particular to try and limit the chance of crossing "tipping points" in the climate system – most of the world's nations have signed up to a non-legally binding target of limiting temperature rise to 2C above pre-industrial levels.

Temperature targets can refer either to the level at which the global temperature stabilises (e.g. levelling off and remaining at 2C) or to a peak (e.g. reaching 2C and then falling to a lower level).

Concentration targets

Humans don't determine the global temperature directly, but by affecting the concentration of greenhouse gases in the air. To meet any temperature target, therefore, the world need to reduce its emissions sufficiently to stabilise that concentration an appropriate level.

Concentration targets can be particularly confusing because "concentration" can refer to three different things:

• the concentration only of carbon dioxide, the main man-made greenhouse gas, as measured in parts of CO2 per million (ppm)• the combined impact of all the man-made greenhouse gases measured in ppm of CO2e (the amount of CO2 that would be required to create an equivalent amount of warming)• the combined impact of man-made greenhouse gases plus the cooling effect of man-made aerosol pollution, also measured in CO2e

To confuse things further, we don't know exactly how sensitive the climate will prove to be to increased greenhouse gas concentration. That means each concentration target could lead to a range of possible temperatures. Picking a concentration target is therefore partly a matter of deciding how much risk we're prepared to take of hitting or exceeding 2C or any other temperature target.

There isn't currently an agreed global target for greenhouse gas concentration, though 450 ppm CO2e (for all greenhouse gases) is frequently cited as a sensible place for the world to aim. Assuming mid-range estimates for climate sensitivity, 450 CO2e ppm would give us a roughly 50% chance of limiting temperatures to 2C according to the last IPCC report. Above that concentration, our chances of avoiding 2C decline; below it, they improve.

Another much-cited target is stabilising CO2 (as opposed to all greenhouse gases) at 350ppm. This would mean reducing the concentration from the current level, which is around 390ppm and rising each year. If this could be achieved – which would most likely entail removing some of the CO2 currently in the air in addition to rapidly reducing emissions virtually to zero – then it might allow global temperatures to level off at around 1.5C above pre-industrial levels, according to the UK's Met Office – though the situation would depend on what happens to emissions of the other greenhouse gases.

The 350 ppm CO2 target was the original rationale for the name of the 350.org campaign – though the group now argues that, due to global warming happening faster than previously thought, the world should instead aim for 350ppm CO2e. This is an even more ambitious target which could see global temperatures stabilised at around 1C above pre-industrial levels, according to the Met Office. Again, though, it's unlikely such a target could be achieved without a significant global effort to remove CO2 from the air.

Emissions targets

Concentrations are still one step removed from direct human control. To meet any temperature or concentration target, we need emissions targets to determine how much greenhouse gas we can we can emit over what timeframe.

Sometimes emissions targets are expressed in terms of relative emissions cuts – for example, cutting global emissions by 50% by 2050, relative to 1990. However, such targets are somewhat broad-brush because they can be met in different ways. Keeping emissions flat until the 2040s and then sharply cutting them would mean far more greenhouse gas entering the atmosphere in total – and therefore a larger temperature rise – than cutting emissions sharply next year and then keeping them flat until 2050.

Less vague than a relative target is an emissions trajectory, which defines how much greenhouse gas the world emits each year going forward. (Even with a trajectory, however, there's still some uncertainty around the resulting concentration, because we don't know exactly what proportion of future emissions will be absorbed by the sea or land, as opposed to remaining in the atmosphere where they cause warming.)

A common way to define an emissions trajectory is to pick a year when global emissions should peak and an annual rate of decline from that peak. Scientists have examined lots of trajectories, each of which gives a range of likely concentrations and in turn a range of temperature outcomes. According to Met Office research, global emissions would need to peak by 2016 and decline by 4% a year to provide a 50% chance of avoiding 2C. For a better chance, we'd need to peak emissions even sooner and/or cut them even faster after the peak.

There is no internationally agreed target for peaking global emissions at present, let alone their rate of decline afterwards. At the Copenhagen climate conference, the draft "Danish Text" included a reference to emissions peaking no later than 2020 (and falling to 50% by 2050). But this text was abandoned and no such goal has subsequently been agreed.

One final type of emissions target is the total amount of CO2 that the world can emit over a particular period – sometimes known as a cumulative emissions target or a carbon budget. According to one landmark study, the world should limit total future emissions to around half a trillion tonnes of CO2 if we want a 50% chance of avoiding 2C.